Within the field of computing, many scenarios involve a content system that presents content items to one or more users, such as images, audio, video, and text. The content items may be presented by insertion into a hosting item, such as a video that is inserted into a content position within a document that is retrieved from a document source, such as a web page retrieved from a webserver. In many such scenarios, when the user initiates a request on a device to present a web page, the device may retrieve the web page from the webserver, and the web page may include an element (such as an HTML tag) that references a content item to be retrieved from a content server and presented at a particular content position. Accordingly, the device may retrieve the content item from the content server, and, within a rendering of the web page that is presented to the user, may present the content item received from the content server.
In some such scenarios, the particular content item to be displayed within the hosting item may be variable; e.g., the content item may comprise a selection of an image from among an image album. The content server may update the image album with new images that may be inserted into the content position of the hosting item. Accordingly, when the device receives the hosting item and initiates the request for a content item to be inserted at the content position, the content server may choose from among the images of the image album and transmit the selected image to the device for inclusion in the hosting item. In this manner, the content item presented within the hosting item may be dynamically updated, and may result in a different presentation of the content item to the user for successive renderings of the hosting item (e.g., to present fresh content items to the user, rather than repeatedly presenting the same content items to the user).